1. Field of the Invention
The present invention relates to a semiconductor device and a method for manufacturing the semiconductor device.
In this specification, a semiconductor device generally means a device which can function by utilizing semiconductor characteristics, and an electrooptic device, a semiconductor circuit, and electronic equipment are all semiconductor devices.
2. Description of the Related Art
A technique by which transistors are formed using semiconductor thin films formed over a substrate having an insulating surface has been attracting attention. The transistor is applied to a wide range of electronic devices such as an integrated circuit (IC) or an image, display device (display device). A silicon-based semiconductor material is widely known as a material for a semiconductor thin film applicable to a transistor. As another material, an oxide semiconductor has been attracting attention.
For example, a transistor whose semiconductor thin film includes an amorphous oxide containing indium (In), gallium (Ga), and zinc (Zn) and having an electron carrier concentration of less than 1018/cm3 is disclosed (see Patent Document 1, for example).
In addition, the transistor including an oxide semiconductor can operate at higher speed than a transistor including amorphous silicon and can be manufactured more easily than a transistor including polycrystalline silicon; however, a transistor including an oxide semiconductor is known to have a problem of low reliability because of high possibility of fluctuation in electrical characteristics.
For example, in an oxide semiconductor element having a top-gate structure and using ZnO as a main component, when silicon nitride (SiNx) formed by a plasma chemical vapor deposition method (a PE-CVD method) is used as a gate insulating film included in an oxide semiconductor film, reductive elimination of ZnO occurs because hydrogen concentration in the silicon nitride film is high, and thus the resistance of the ZnO layer is lowered. Against the problem, reducing hydrogen concentration in the gate insulating film on the side in contact with the interface with the oxide semiconductor film and using a gate insulating film having a two-layered structure including two films with different hydrogen concentrations are disclosed (see Patent Document 2).